Investigators fear that the British-born bomber Salman Abedi, a 22-year-old of Libyan descent, was part of a wider network of Isil-inspired terrorists, including a bomb-maker, who may still be at large.

Special Forces were deployed to Manchester ready to engage in the hunt for accomplices of Abedi, who killed 22 concert-goers in Britain’s worst terrorist atrocity for 12 years.

Outlining the increased risk, Mrs May said: “It is a possibility we cannot ignore that there is a wider group of individuals linked to this attack.”

It is the first time Britain has been on maximum terrorist alert since 2007, when a blazing car loaded with gas canisters was driven into Glasgow Airport.

A 23-year-old man was arrested in Manchester in connection with the attack and was being questioned.

Intelligence experts believe the device detonated at a concert by the pop singer Ariana Grande at the Manchester Arena on Monday night was so sophisticated that Abedi must have either been given specialist training abroad or used a bomb made by a technician who has not yet been captured.

It emerged that Abedi had travelled to Libya, raising fears he had been trained there and posing questions for the security services on whether he should have been tracked. Police confirmed that an eight-year-old girl was among those killed.

Grande has a huge following among young girls and other children are understood to be among the dead. Twelve children were among 59 taken to hospital after the atrocity, for which Isil has claimed responsibility.

Mrs May described the bombing as “one of the worst terrorist incidents we have ever experienced in the United Kingdom” and said it “stands out for its appalling, sickening cowardice – deliberately targeting innocent, defenceless children and young people who should have been enjoying one of the most memorable nights of their lives”.

The first victims of the attack were named as Saffie Rose Roussos, eight, Georgina Callander, 18, and John Atkinson, 26;

Saffie’s mother, Lisa, is reportedly among those seriously injured in hospital and does not yet know her daughter is among the dead;

Vigils for the dead and wounded were held in Manchester and other cities across the UK;

The Queen described the attack as “barbaric” in a message of support to the people of Manchester;

Campaigning for the election was suspended “until further notice”.

Sources confirmed to The Daily Telegraph that Special Forces soldiers had been sent to Manchester on standby to support the police in the hunt for other potential bombers.

Military bomb disposal experts were already involved in the operation on Tuesday when police stormed the house in the Fallowfield area of Manchester where Abedi is believed to have lived with relations.

His family fled Libya to escape Muammar Gaddafi’s regime, but his father moved back there after the fall of Gaddafi during the Arab Spring in 2011.

The intelligence services are urgently investigating whether Abedi had links to known extremists in Libya. A number of Libyans who have spent time in Manchester before moving back to north Africa are known to have connections with al-Qaeda and Isil.

Air Marshal Sir Stuart Peach, the Chief of the Defence Staff, told the Cobra committee he was ready to put armed soldiers on the streets if directed by the Prime Minister.

Explosions at The Manchester Arena on Monday eveningCredit:
Eamonn and James Clarke

Mrs May said of the attack: “We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish, but as an opportunity for carnage.

“But we can continue to resolve to thwart such attacks in future, to take on and defeat the ideology that often fuels this violence, and if there turn out to be others responsible for this attack, to seek them out and bring them to justice.”

Police raid a house on Elsmore Rd in Fallowfield, South Manchester Credit:
Eamonn and James Clarke

President Trump said the bomber and any accomplices were “evil losers in life”. He said: “I won’t call them monsters because they would like that term. I will call them losers.”

The Russian president Vladimir Putin, China’s President Xi and Germany’s Chancellor Merkel were also among those who sent messages to the Prime Minister.

Later this week Mrs May will travel to the G7 summit in Sicily where she will lead discussions on anti-terrorism measures with other leaders. A Downing Street spokesman confirmed the trip will go ahead and that Mrs May will play a major role in sessions about counter-terrorism cooperation.